Integrated circuit micro-electromechanical system (MEMS) devices can require specialized packages or packaging processes. Some MEMS devices, such as pressure sensors, often require specialized packaging solutions to accommodate one or more external pressure ports because at least a portion of the sensor chip surface needs to be exposed to the ambient environment without the common protection of mold compound and/or a package wall. On the other hand, bond pads and bond wires of the sensor die present critical challenges with respect to external media compatibility and need adequate protection. These opposing needs present challenges for the design of package solutions, often leading to specialized solutions that are not standardized, necessitating changes to the wafer and assembly processes that are complicated and expensive.
For example, conventional MEMS pressure sensors often have a premolded cavity package, with the cavity opened or exposed on one side for the sensor die and bond wires. The chip surface with the sensing element faces the open portion of the cavity package. The sensor die and bond wires can be covered with a gel or other suitable material filled into the cavity package to protect the bond pads, bond wires and the sensing elements from external media. One disadvantage of such a conventional package design is that the gel often cannot provide adequate protection against aggressive media. As previously mentioned, this is most critical for the sensitive bond pads and bond wires. Another disadvantage is the relatively high material cost for the premolded package, leading to higher product cost. Further, this package type is not compatible with relative pressure sensor applications, discussed in more detail below.
Another conventional solution is to use a standard leadframe and to form a cavity during the mold process with a specially shaped mold tool. In this case the sensor die bonding into the formed cavity and the wire bonding are done after the mold process. This approach also usually requires a protective gel on the sensor die. The previously mentioned disadvantages relating to media compatibility and the incompatibility for relative pressure sensor applications are also applicable here.
Yet another package solution is to use a leadframe with a predefined pressure port in combination with a specially shaped mold tool to create a pressure port, for example at the backside of the package. In this case the sensing element faces the backside of the sensor chip in order to create a pressure port by attaching the sensor die with the sensing element on top of the pressure port in the package backside. The bond pads and the bond wires on the frontside of the chip can be covered with the mold compound. A disadvantage of this approach is a higher package cost compared with that of a standard package. Special and cost-intensive MEMS processes are required at the wafer process to create the MEMS structure with the backside opening on the sensor die. Absolute and relative pressure sensing applications require different wafer and assembly processes in this case, which is not desired.